The present invention relates to an improved method for processing pig iron containing vanadium to produce a valuable slag having a high ratio of vanadium to iron which is useful in the production of vanadium alloys.
Vanadium in the form of its low-melting ferrovanadium alloys, is widely used as an alloying agent to impart toughness and other desirable properties to iron and steel. Among the most useful vanadium alloying agents are the alloys produced according to the process of Rathmann and Rasmussen U.S. Pat. No. 3,420,659 issued Jan. 7, 1969. This process is carried out in two steps. In the first step a vanadium-containing material is smelted with silica, a flux, and a carbonaceous reducing agent to produce a primary vandium silicide having a silicon content of 25 to 60 percent. Inasmuch as such a silicon content is too high for an alloying agent for iron or steel, the primary silicide is refined in the second step of the process by melting it with lime and a vanadium-containing refining agent to increase the vanadium content of the alloying agent and reduce its silicon content to less than 20 percent. While the richest source of vanadium for the refining step is vanadium pentoxide, this material is very expensive due to the chemical processing required in its manufacture. For that reason it is preferred to look elsewhere for the necessary vanadium. A number of relatively inexpensive sources of vanadium exist such as spent catalysts, petroleum residues and vanadium-bearing slags from the refining of pig iron.
To be useful in the refining step of the Rathmann et al. process, a refining agent should contain at least about 8 percent of vanadium. Materials containing up to about 34 percent vanadium may be used with advantage. Materials having a higher content can be economically processed by aluminum reduction and so are not used in this process normally. Raw materials containing less than 8 percent or more than 34 percent vanadium may be blended to produce a refining agent having a suitable concentration. It should be noted here that while throughout this specification and appended claims vanadium content of the various materials, such as ore, slag, pig iron or other material, is expressed in terms of metallic vanadium, those skilled in the art will appreciate that the vanadium values may be present in various forms other than metallic vanadium, such as V.sub.2 O.sub.3, V.sub.2 O.sub.4, V.sub.2 O.sub.5 or some other vanadium compound.
The preferred refining agents for use in the Rathmann et al. process are vanadium-containing slags derived from the refining of vanadium-containing pig iron. Such slags are available in huge quantities and generally contain more than the required 8 percent of vanadium. However, it is also necessary to the process that the slag have a vanadium to iron weight ratio of at least about 0.5, and preferably from about 0.75 to 1.0, to be useful directly as a refining agent without prior deironing. Available slags are generally deficient in this respect. For example, the South African Highveld slags contains 12 to 16 percent vanadium, but their vanadium to iron ratio is less than 0.5, typically only about 0.4.
In the Highveld process titaniferous magnetite ore from the Bushveld igneous complex of South Africa, containing about 1 percent vanadium, is fed into large prereduction kilns together with appropriate amounts of coal, dolomite and silica and heated to about 1830.degree. F. In this way the oxygen content of the iron ore is reduced to about 70 percent of theoretical, the coal is charred, and the dolomite is calcined. The hot partially reduced burden of the kilns is conveyed in refractory lined hoppers to bins feeding the smelting furnaces. This material is then smelted in submerged arc electric furnaces operated at a power input level of 20 to 25 megawatts to produce hot metal which has the following typical analysis: 3.5 percent carbon, 1.28 percent vanadium, 0.25 percent silicon, 0.16 percent titanium, 0.065 percent sulfur and 0.075 percent phosphorous. The slag from the smelting operation generally analyzes: 20 percent TiO.sub.2, 18 percent CaO, 17 percent MgO, 19 percent SiO.sub.2 and 13 percent A1.sub.2 O.sub.3.
The resulting hot metal is blown with oxygen in a shaking ladle at a temperature maintained below 2550.degree. F. with the addition of scrap and ore as necessary. The resulting blown metal contains only traces of metalloids and about 3.1 percent carbon. The slag from the shaking ladle oxygen blowing operation contains the bulk of the vanadium, typically about 14 percent vanadium by weight but, as noted above, the vanadium to iron ratio of such Highveld slag is less than the level of 0.5 needed in the refining operation of the Rathmann et al. process, being typically about 0.4. For this reason, it is necessary to beneficiate or deiron such slags by melting them with lime and treating the melt with sufficient primary silicide from the process to reduce a substantial part of the iron oxide to metallic iron which is separated to increase the vanadium to iron ratio of the slag. This beneficiation process also oxidizes the vanadium, silicon, titanium, manganese and chromium values of the primary silicide and incorporates them in the deironed slag. In view of the foregoing it is apparent that there has been a long standing need in the art to provide an inexpensive vanadium-containing refining agent for use in the second step of the Rathmann et al. process containing from about 8 to 34 percent vanadium and having a vanadium to iron ratio of at least about 0.5, and that it would be especially desirable to provide for this purpose a slag from the refining of pig iron which would meet the necessary criteria without the need for further beneficiation.
It is, therefore, a primary object of the present invention to provide an improved process for the direct production, from vanadium-containing pig iron, of a slag which contains at least 8 percent vanadium and which has a vanadium to iron ratio of at least about 0.5.